1. Field of the Invention
This invention relates to a method of operating a hybrid electric vehicle to reduce tailpipe emissions and more particularly, to a method of operating a hybrid electric vehicle which utilizes the vehicle's electric motor/generator to reduce emissions during cold-start and transient conditions.
2. Background Art
Conventional vehicles having internal combustion engines utilize a three-way-catalyst (“TWC”) to reduce tailpipe emissions. Particularly, the TWC catalytically reduces nitrogen oxides (NOx) and oxidizes carbon monoxide (“CO”) and unburned hydrocarbons (“HC”) which are produced during the combustion process. The TWC has a very high conversion efficiency once the catalyst has “warmed up” and the air-fuel ratio of the mixture is near its stoichiometric point. An example of the conversion efficiency of a typical catalytic converter over time is shown in graph 100 of FIG. 5.
In conventional vehicles, more than fifty percent (50%) of the HC and CO emissions are generated in the first sixty seconds of a standard emissions test cycle (e.g., the EPA75 test cycle), and more than twenty five percent (25%) of the NOx emissions are generated during that time. An example of the tailpipe emissions of a vehicle during a standard emissions test is shown in graph 110 of FIG. 6. The point in time at which the catalytic converter reaches a fifty percent (50%) efficiency is commonly referred to as its “light-off” time. Due to the relatively poor efficiency of the catalytic converter prior to “light-off”, recent efforts to reduce tailpipe emissions have concentrated on reducing the “light-off” time, thereby reducing the time during which the catalytic converter is least efficient. These prior efforts have also included concomitantly altering the air-fuel ratio and/or retarding the spark calibration of the engine.
These prior efforts have suffered from some drawbacks. Particularly, the difficulty in controlling the combustion stability of the engine and the operating load of the engine as it warms up severely limits these prior strategies. Moreover, although significant fractions of the emissions are produced during “cold start” type conditions (e.g., during the first sixty seconds of vehicle operation), periods when engine operating loads are changing rapidly or “transient events” also produce a significant portion of the emissions, specifically NOx emissions (see e.g., FIG. 6). Hence, these methods which concentrate on cold-start type operating conditions do not adequately address or improve emissions during transient events once the vehicle has warmed up.
Hybrid electric vehicles have been designed and manufactured for the purpose of improving fuel economy and emissions. Particularly, hybrid electric vehicles utilize both an internal combustion engine and one or more electric motors to generate power and torque. The electric motor(s) within a hybrid electric vehicle provides the vehicle with additional degrees of freedom in delivering power and torque. While hybrid electric vehicles significantly reduce emissions, the foregoing emissions reducing strategies are not well-suited for use with hybrid electric vehicles. Particularly, the foregoing emissions reducing strategies do not maximize and/or utilize the flexibility of hybrid electric vehicles to utilize both an electric motor and an internal combustion engine to provide power and torque.
There is therefore a need for a method of operating a hybrid electric vehicle to reduce emissions which overcomes the drawbacks of prior emissions reducing methods, strategies and systems.